German Patent Publication (DE-PS) 3,117,971, corresponding to U.S. Pat. No. 4,505,206 (Gottzein et al.), published Mar. 19, 1985, discloses a control system for adapting the air gap width between electromagnetically levitated vehicles and a rail, to an optimal performance. The known control loop contains nonlinear elements and essentially equalizes intermittent, short duration disturbances. For this purpose, the rated air gap setting is increased on route segments with large roadway disturbances. This type of control has the disadvantage of so-called constant-clearance control systems that keep the air gap width at a constant width determined also by the frequency of roadway disturbances. This feature causes a hard coupling between the vehicle and the rail segments. The action of a hard coupling is comparable to that of a hard mechanical spring, the static spring rate or stiffness of which is infinitely high. Another disadvantage in the known control system is that it does not provide any acoustic damping in the low and medium sonic frequency ranges.
In ships or submarines, electromagnetic levitation systems equipped with a constant air gap width control as disclosed in the above mentioned reference, transmit machine-related vibrations of, e.g., power supply units, prime movers or auxiliary power units to the machine bed and hence to the hull, which involves the risk of sonar detection and damage to the structure. In other vibration-sensitive structures, such as aircraft, buse, railroad trains, steel structures, or reinforced concrete structures, a constant air gap width control of the electromagnetic levitation system for machines, causes a high structure-borne noise coupling in the low and medium sonic frequency ranges thereby producing disadvantageously high noise levels.